This invention relates to a hollow fiber membrane fluid separation device which is adapted for boreside feed of a fluid mixture and which contains multiple concentric stages.
Hollow fiber membrane devices commonly contain two or three regions, wherein such regions are sealingly engaged or otherwise maintained separate and distinct so that fluid cannot communicate from one region to the other, except by passing the fluid through the bores of the hollow fibers or by permeating the fluid across the walls of the hollow fibers. Generally, a hollow fiber membrane device comprises a bundle of hollow fibers arranged in a fashion such that each end of the hollow fibers are embedded in a resin matrix commonly referred to as a tubesheet or header. Such hollow fibers communicate through the tubesheets and are open on the exterior face of each tubesheet. The exterior face of the tubesheet as used herein means the face of the tubesheet which is opposite the bundle.
Generally, the regions of a hollow fiber membrane separation device are maintained separate and distinct by the tubesheets and seals about the tubesheets. In a shellside feed process, the region about the outside of the hollow fiber bundle between the tubesheets is pressurized. The fluid mixture to be separated is introduced into the device in the region between the tubesheets and the outside of the hollow fibers, that is, the shellside, and the fluid which permeates through the hollow fiber membranes into the bores of the hollow fibers is removed at one or both ends of the hollow fibers in the region(s) adjacent to the exterior face of one or both tubesheets. The non-permeating fluid is removed from a region in the area between the tubesheets and the outside of the hollow fibers. Most commercial industrial fluid separation membrane devices and processes operate in this fashion.
In a boreside feed process, the mixture of fluids to be separated is introduced into one end of the hollow fiber membrane device adjacent to the exterior face of the first tubesheet such that the fluid mixture flows down the bores of the hollow fibers through the first tubesheet and into the portion of the hollow fibers contained in the region between the tubesheets. In the region between the tubesheets, the fluid which selectively permeates through the hollow fiber membranes is removed from the shellside of the device on the outside of the hollow fibers. The fluid which does not permeate through the hollow fiber membranes exits into a region adjacent to the exterior face of the second tubesheet and is removed from that region. In such a boreside feed operation, pressure is exerted on the exterior faces of the tubesheets which are opposite the fiber bundle. The bores of the hollow fibers are also pressurized in such an operation. As the tubesheets are usually comprised of a resinous material, significant bending, compressive, and shear stresses are exerted on the tubesheets by such a boreside feed operation. Such stresses exerted on the tubesheets create a problem with respect to supporting the tubesheets and preventing the tubesheets from collapsing in on the hollow fiber bundle.
A second problem associated with boreside feed is obtaining adequate flow distribution of the permeate on the shellside of the hollow fiber membrane device so that efficient separation can be achieved. One of the driving forces for transport through the membrane is the concentration gradient across the membrane. As the fluid mixture to be separated flows down the bores of the hollow fibers and the selectively permeable fluid permeates through the hollow fibers, the concentration of the selectively permeable fluid inside the hollow fibers is reduced and the concentration of the selectively permeable fluid on the shellside (outside) of the hollow fibers increases. This results in a decrease of the concentration driving force across the membrane, which lowers separation performance.
A third problem associated with boreside feed is that if the flow on the shellside of the hollow fiber membranes is not properly controlled, the shellside of the hollow fiber membrane device will contain localized areas of high concentration of the permeate fluid, which further reduces separation performance.
Yet another problem encountered, particularly in the separation of gases, is the inability to obtain a non-permeate product stream of high purity with a single membrane device, due to inefficiencies associated with conventional membrane devices which are single stage. Series operation using multiple membrane devices has therefore been necessary to achieve a high purity non-permeate product stream. Such use of multiple membrane devices to achieve a high purity non-permeate product stream is costly due to the use of multiple cases and the additional piping required for multiple membrane devices. The space occupied by such multiple membrane devices is also a disadvantage.
What is needed is a hollow fiber membrane fluid separation device which is adapted for boreside feed and minimizes the stresses on the tubesheets. What is further needed is such a membrane device in which the flow of the permeate on the shellside of the device is controlled to maximize the concentration gradients along the hollow fibers and to prevent localized areas of high permeate concentration, thus enhancing the flow of permeate on the shellside of the device and rendering the device more efficient. What is further needed is such a membrane device which is capable of producing a high purity non-permeate product stream which is less costly and occupies less space than conventional systems utilizing multiple membrane devices.